The invention relates to a switched-mode power supply capable of operating optionally in two states, comprising at least two voltage sources and having at least two supply output terminals and comprising first means for supplying a signal which acts, on the switching operation, to reduce all the voltages supplied by the sources when one voltage exceeds a first reference value.
The invention also relates more specifically to a circuit capable of operating optionally in the following states:
either in a first state in which a first voltage source connected to a first output terminal applies thereto a first voltage, and at least a second voltage source connected to at least a second output terminal applies thereto a second voltage which is higher than the first voltage,
or in a second state in which a voltage which is higher than or equal to the voltage applied to the first terminal in the first state is applied to it, while a voltage is applied to the second terminal which is less than the voltage applied to it in the first state.
The circuits of the type according to the invention are used particularly in television receivers having a remote control. They may be in two states corresponding to either normal operation of the television receiver or to the stand-by state, in which the receiving circuits of the remote control are normally fed, while the other circuits are supplied with a reduced voltage or are not fed at all.
In widely circuits, two separate supply sections are used, one for the remote control receiving circuit, the other one for the remaining portion of the television receiver. This system is expensive as it requires two separate power supplies.
In another widely used circuit, one single switched-mode power supply is used, and the voltages, other than those feeding the remote-controlled receiver, are cut-off, by means of change-over switches, during the stand-by state. It is then necessary to provide the possibility to cut-off, during operation, several high voltages and currents, which necessitates the use of a plurality of highly reliable and therefore expensive interruptors.
German Patent Specification No. DE 2,620,191 discloses a more satisfactory circuit, in which one of the windings of a switched-mode supply transformer is wound in a sense opposite to the winding sense of the other windings. Consequently, the voltages from this winding do not depend on the duty cycle. By varying the duty cycle, it is possible to reduce the other voltages without modifying the voltage from the said winding, which is used to feed the remote control receiving circuit. This system has, however, the disadvantage that the voltage from the inverse winding is not stabilized by the overall stabilizing system and that it is then necessary to provide an additional series-arranged stabilizing circuit for the usual case in which the line voltage may vary considerably, and especially with the now prevailing tendency according to which the television sets may be connected without change-over to all the 90 v. to 250 v. lines voltages. This stabilizing circuit, apart from its price, has the disadvantage that it increases the power consumption in the stand-by state.